It is well known that there are many lighting systems whose dependability and continuity of operation is essential. Thus, for example, lighting in hospital operating rooms and other critical areas such as emergency exits must be maintained during power outages that sometimes unavoidably occur. In addition, there is a vital need for the maintenance of lighting in other important but somewhat less critical areas such as manufacturing facilities where loss of lighting results in shut-down, thus requiring evacuation of personnel, and entailing unacceptable financial/economic detriment.
Because of the necessity to ensure continuity of at least a minimum threshold level of lighting in critical areas, and because of the economic importance of maintaining normal levels of lighting in other areas, a variety of proposals have heretofore been made. These have included the provision of separate stand-alone emergency lighting units critically positioned so as to provide sufficient light to permit orderly evacuation, separate motor-generator sets adapted for start-up to provide power during emergencies, wholly or partly redundant lighting units, and/or battery powered emergency sources adapted for switching during emergencies so as to provide temporary sources of power during brief outages. However, the provision and maintenance of equipment necessary for the practice of these proposals has in some instances been excessively costly, in others bulky or cumbersome, in others essentially duplicative, or in others insufficiently reliable or dependable. Accordingly, there has continued to be a need for an improved and relatively inexpensive lighting system that provides a fully dependable and continuous supply of normal lighting.